Galvannealed steel sheets are low price, have excellent rust prevention property, and therefore are widely used as automobile steel sheets. The galvannealed steel sheet is required to have not only excellent corrosion resistance, but also a good surface appearance, powdering resistance, and friction property upon press forming.
Poor surface appearance in the GA includes non-coating, ripple, and galvannealing non-uniformity. The non-plating means that a non-coating portion exists on the steel sheet, which should be avoided since the appearance is damaged, and the rust prevention property is adversely affected. It is conventionally known that the non-coating is easily produced when an alloy element such as Si, Mn and P is increased for strengthen the steel sheet, these strengthen elements are produced on the surface of the steel sheet as oxides in annealing prior to coating, to decrease wettability between the steel sheet and zinc.
Even if the coating is deposited on the steel sheet, a too large amount of the coating is deposited on a portion where the coating is considered to be deposited together with an oxidized film on a surface of a coating bath. Such portion has a different color from other portions, and is convex. As a result, appearance non-uniformity is observed, and is referred to as the ripple. In a galvannealing treatment, the portion where the oxides are deposited has a different galvannealing rate from those of the other portions. The portion has the larger amount of the plating, and has a convex surface so that the portion is in a white color, which is different from that of the other portions. The ripple is easily produced when strengthen elements are increased, similar to the non-coating. It is considered that the ripple is produced by an effects of the oxide of the strengthen element produced on the surface of the steel sheet so that the oxidized film on the surface of the coating bath is easily deposited on the steel sheet.
The galvannealing non-uniformity is produced by a difference in galvannealing rates. A difference in color is produced on the GA surface since a not-galvannealed portion remains. An irregular color appearance is observed. The galvannealing rate largely depends on a galvannealing temperature and an Al concentration in the coating bath.
On the other hand, coating layer properties largely depends on the press formability of the galvannealed steel sheet. In the GA, a Zn—Fe alloy coating phase is produced by a diffusion of zinc and steel sheet (Fe). A Γ phase (including a Γ phase and a Γ1 phase) is produced at a steel sheet side of the coating layer, and a ζ phase is produced at the surface of the coating layer. The Γ phase has high Fe content, and is hard and brittle, which inhibits tight coating adhesion, and especially becomes a factor of a coating peel, which is called powdering, upon the press forming. The ζ phase is soft, which inhibits the friction property upon the press forming, and becomes a factor of a press crack.
Conventionally, a number of attempts have been made in order to improve the surface appearance and the press formability as described above.
For example, as to non-coating and the ripple caused by the decrease in the wettability between the steel sheet and zinc, Japanese Unexamined Patent Application Publication No. 7-70723 proposes a method for coating by concentrating components in a steel sheet on a surface of the steel sheet with annealing, removing a layer thus-concentrated with pickling, and then heating again. However, since the method needs two times of annealing and pickling steps, the costs inevitably increase.
As to the galvannealing non-uniformity, Japanese Unexamined Patent Application Publication No. 5-132748 proposes a method for regulating the amount of Al in the bath by the amount of Ti and P in the steel. However, the contents of the elements in the steel differ depending on a tapping steel. It is extremely difficult to change the amount of Al in the bath in response thereto. It will also be disadvantage in the cost point of view.
In order to improve the non-coating, the galvannealing non-uniformity, and the powdering resistance, Japanese Unexamined Patent Application Publication No. 6-88187 proposes a method for forming a metal coating layer made of Fe, Ni, Co, Cu and the like on a steel sheet after annealing but before coating. However, a normal continuous galvannealing line includes no facility to produce the metal coat after the annealing and before plating. It requires to newly provide the facility. It is difficult to conduct the method that requires the coat forming process.
As to the friction property improvement, Japanese Unexamined Patent Application Publication No. 1-319661 discloses a method for iron-based electrogalvanizing on an upper layer of a galvannealed steel sheet. However, in the method, the electrogalvanizing step is needed extra in addition to the normal production steps of the galvannealed steel sheet. It makes the steps complex, and increases the costs.
As to the powdering resistance and friction property (stability of a friction coefficient within a coil) improvement, Japanese Unexamined Patent Application Publication No. 9-165662 indicates that a high temperature galvannealing at 495° C. or more and at 520° C. or less, with a bath temperature of 470° C. or less, a high immersed sheet temperature, whereby a production of a soft ζ phase is inhibited and galvannealing is performed microscopically to provide excellent powdering resistance. Japanese Unexamined Patent Application Publication No. 9-165663 indicates that the similar effects are obtained by a low bath temperature of 460° C. or less, and a high temperature galvannealing at 495° C. or more and 520° C. or more.
However, in the operation in which the bath temperature and the immersed sheet temperature is different, the coating bath temperature is not stabilized, and a production of a dross is increased by a change in the bath temperature and a bath temperature difference between a steel sheet and the other portions. The dross is attached to the steel sheet, resulting in a poor appearance. When the steel sheet is immersed in the bath at high temperature or at low temperature, the bath temperature increases or decreases by a heat transfer between the steel sheet and the coating bath. In order to stabilize the bath temperature, it is required to provide a temperature control device and the like for cooling or heating the coating bath at lower or higher than the normally required.
Thus, the conventional methods for improving the surface appearance and the press formability of the galvannealed steel sheet unfavorably requires new steps and facilities, and lacks the stability in the coating operation.
An object of the present invention is to provide a galvannealed steel sheet with excellent surface appearance and press formability, and its production method, that can solve the aforementioned conventional problems upon the galvannealed steel sheet production.